Impact Toughness of Subzones in the Intercritical Heat-Affected Zone of Low-Carbon Bainitic Steel

Abstract

The subzones of the intercritical heat-affected zone (IC HAZ) of low-carbon bainitic steel were simulated by using a Gleeble-3500 simulator to study the impact toughness. The results showed that the IC HAZ is not entirely brittle and can be further divided into three subzones according to the impact toughness or peak welding temperature; the invariant subzone heated between the critical transformation start temperature (Ac1) and 770 °C exhibited unchanged high impact toughness. Furthermore, an extremely low impact toughness was found in the embrittlement subzone, heated between 770 and 830 °C, and the reduction subzone heated between 830 °C and the critical transformation finish temperature (Ac3) exhibited toughness below that of the original metal. The size of the blocky martensite-austenite (M-A) constituents was found to have a remarkable level of influence on the impact toughness when heated below 830 °C. Additionally, it was found that, once the constituent size exceeds a critical value of 3.0 µm at a peak temperature of 770 °C, the IC HAZ becomes brittle regardless of lath or twinned martensite constitution in the M-A constituent. Essentially, embrittlement was observed to occur when the resolved length of initial cracks (in the direction of the overall fracture) formed as a result of the debonding of M-A constituents exceeding the critical Griffith size. Furthermore, when the heating temperature exceeded 830 °C, the M-A constituents formed a slender shape, and the impact toughness increased as the area fraction of the slender M-A constituents decreased.

Investigation into the Impact Toughness of API 5L X80 Steel Weldments and its Relationship with Safe Welding Procedures

“…The effect of welding on the microstructure of the HAZ, is characterized by significant changes at different distances from the fusion line ( Figure 5). The original microstructure consisting of ferrite and pearlite, also observed for 3.0 mm from the fusion line (3FL), is modified and lath and granular bainite are observed closer to the fusion line (1FL), which is consistent with other investigations 18,21,[65][66][67] . At 2.0 mm from the fusion line, lower microhardness values (~10%) compared to the base metal are observed, both in the as welded and heat treated conditions.…”

“…This already happens for ASME IX 77 , when considering the chemical composition, welding position, etc. In this case, the Charpy-V notch can be positioned as close as possible to the fusion line as suggested by the API-6A 45 , in complement to the positions required [44][45][46][47][48][49][50][51][52] , to increase the chance to contemplate the critical region of the HAZ, as reported in the literature 7,14,16,[18][19][20][21] and confirmed by this work.…”

“…In addition, it contributes to show the complexity of the problem if evaluated by simulation experiments. For example, Li et al 21 suggest that there is a critical size for the MA constituent such that a controlled cleavage fracture is produced and suggested that the detrimental effect of MA with a size less than 3μm can be ignored. However, this value is superior to the one found in this work (Figure 10).…”

“…To use these pipes safely, it is necessary to evaluate the welding effects on the heataffected zone (HAZ) and weld metal properties carefully 14 . Regions presenting poor impact toughness in the HAZ of API X80 welded joints, known as local brittle zones (LBZ), have sometimes been observed 7,10,11,[14][15][16][17][18][19][20][21] . Usually, the LBZ occurring in the ICCGHAZ are associated to the presence of MA constituents.…”

“…Usually, the LBZ occurring in the ICCGHAZ are associated to the presence of MA constituents. Due to these observations there is extensive information addressing the effect of the morphology, size and distribution of MA constituents on the properties of the HAZ available in the literature 7,10,11,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] Several studies have reported that the amount of MA constituents is the primary metallurgical factor affecting the impact properties [24][25][26][27][28][29][30] . Some works describe the deleterious effects of large volume fraction of the MA constituents on the impact toughness 27,28 .…”